fix: LinkSettings UI updating when link is disconnected

Comm Links UI was not updating the enabled property of the Disconnect button after
being successfully disconnected, this is due to shared_ptr being automatically cleaned
up and no signal back to the UI on the change of the link state. Addition of signal
emission from qml disconnect method is a bandaid that works.

src/ui/preferences/LinkSettings.qml

@@ -130,7 +130,10 @@ Rectangle {
         QGCButton {
             text:       qsTr("Disconnect")
             enabled:    _currentSelection && _currentSelection.link
-            onClicked:  _currentSelection.link.disconnect()
+            onClicked:  {
+                _currentSelection.link.disconnect()
+                _currentSelection.linkChanged()
+            }
         }
         QGCButton {
             text:       qsTr("MockLink Options")